An electric double layer capacitor has characteristics not seen in batteries, for example, the electric double layer capacitor is rapidly chargeable or dischargeable, highly resistant against overcharge and over-discharge, ensured with a long life because of no involvement of a chemical reaction, usable over a wide temperature range and friendly to environment by containing no heavy metal. Therefore, the electric double layer capacitor has been heretofore used for a memory backup power source or the like. Moreover, with recent abrupt progress of large capacitance capacitors, development of use for high-performance energy devices is proceeding and studies are also being made to use the capacitor for an electric power storage system, an engine assist of hybrid cars or the like by combining it with a solar cell or a fuel cell.
The electric double layer capacitor has a structure such that a pair of positive and negative polarizable electrodes each formed of an active carbon or the like are faced with each other through a separator in a solution containing an electrolyte ion. When a direct current voltage is applied to the electrodes, anion in the solution is drawn to the electrode polarized to the positive (+) side and cation in the solution is drawn to the electrode polarized to the negative (−) side, whereby an electric double layer is formed on the interface between the electrodes and the solution and this is used as an electric energy.
Conventional electric double layer capacitors are excellent in the power density but, on the other hand, disadvantageous in that the energy density is inferior. In order to realize use for energy devices, capacitors having a larger capacitance must be developed. For increasing the capacitance of an electric double layer capacitor, it is inevitable to develop an electrode material of forming many electric double layers between the electrodes and the solution.
Accordingly, use of an active carbon having a large specific surface area has been studied with an attempt to form a larger number of electric double layers. However, the active carbon has a problem that the electrode density decreases and therefore, the electric capacitance per volume (F/ml) does not increase as expected, despite excellent electric capacitance per mass (F/g).
In recent years, a technique of producing an active carbon having a microcrystal analogous to graphite and using it as a raw material of the polarizable electrode has been proposed (see, for example, JP-A-11-317333). This active carbon is excellent in that the electric double layer capacitor using it as a raw material of the polarizable electrode has a large electric capacitance (F/ml).
An active carbon obtained by heating a pitch-originated carbon material in the co-presence of an alkali metal hydroxide and thereby activating the material (alkali activation) has been proposed (see, for example, JP-A-5-258996). Also, an electric double layer capacitor having a high electrode density and a large electric capacitance per unit volume has been proposed, which is obtained by alkali-activating a carbon material having a comparatively high crystallinity, so-called mesophase pitch (see, for example, JP-A-10-121336).
However, these active carbons also have a problem and are not satisfactory. More specifically, the active carbon of JP-A-11-317333 expands when a voltage is applied and the cell may be damaged due to expansion. For preventing the expansion, a dimension-limiting structure is necessary and this gives rise to a large problem in the operation of assembling a capacitor. In addition, the electric capacitance is not expressed unless a voltage of about 4 V is previously applied and therefore, the electrolytic solution may be decomposed.
The active carbons of JP-A-5-258996 and JP-A-10-121336 have a problem in that despite large electric capacitance (F/g), the electrode density decreases due to excessive growth of pores and a small electric capacitance (F/ml) results.